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Coating Selection for Parts in
Sliding or Rolling Contact with
Another

If your part is in sliding or rolling contact with another engineering component then, even if it is lubricated, there is the likelihood of adhesive wear.

Before continuing with the analysis, you should go to the "Problem Solver" (see side bar) and read the sections on wear, friction and lubrication.

Adhesive wear can occur in many engineering situations; for instance shafts, journals, pistons and rings, cams, bearings, pads, gears, seals, slideways, etc., as well as in metal cutting, drawing and forming. In general, if both mating parts are metallic, it will be the softer of the two that suffers the greater wear and it should be that component which is surface engineered. However, there are cases where replacement of a particular part is difficult and it's then that part which should be protected, even at the expense of extra wear on the mating surface.

If the mating part is non-metallic, wear could still occur on your component. If the counterface is plastic, then you should determine if it has any fillers which could cause abrasive wear. This would also be the case if the counterface was a ceramic. Both these areas are covered in this section.

The base material

This could be cast iron, low carbon steel, medium carbon steel, alloy steel (tool steels and bearing steels), stainless steel (austenitic or ferritic), aluminium alloy, titanium alloy or non-ferrous metals like bronze, copper or brass. The analysis below takes this base material as the first selection consideration, but remember that the surface treatment and the substrate are a system.

Contact conditions

The important questions are:

  1. Is the part lubricated? - If so, wear (even without surface engineering) might be a factor of 1,000 less than if it was running unlubricated. In high load/high speed lubricated systems (e.g. cams and tappets, piston bores and rings) there is the possibility of scuffing.
  2. Do you need to reduce friction? - This will only usually be the case for dry sliding and it is important to specify the exact requirements. A low friction coefficient can be defined as 0.1 or less and this can generally be achieved with polymers like PTFE. Such coating will not give low wear rates. If, without surface engineering, the friction would be unacceptably high (e.g., galling between two soft steel parts), then nearly all treatments will reduce that friction as well as reducing the wear.
  3. Is the specific loading low or high? - Loads above 100N/mm2 can be considered as high, in which case the hardness and thickness (or case-depth) of the surface treatment is the critical factor. Both the substrate and the coating must be able to withstand that load. Rolling parts are often under high specific loading.
  4. What are the requirements for reducing wear? - In general, the higher the hardness of the surface layer, the lower will be the wear. It is important to establish the critical aspects of the wear situation. For instance, it may be the increase in clearance between two parts that is the main concern. If wear is concentrated in a small area, then even a low wear rate will lead to a rapid increase in the clearance and you need a high surface hardness. If the wear is spread out over a wide area, the corresponding increase in clearance will be smaller and you may be able to adopt an easier or cheaper solution.
  5. Is there an element of corrosion? - For instance, if moisture or salt water is present there is a major risk of combined wear and corrosion which can rapidly increase surface material loss. In that situation you would look for coatings with good corrosion resistance rather than high hardness.

Surface Engineering Options

Select your substrate or your specific operating conditions from the list below:

Specific Contact Situations

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